Study On Functional Modification Of Spinel Structure(Co_1-xNi_x)₃O₄ Nanowires And Electrocatalytic Oxygen Evolution Reaction Performances

Hydrogen is the only energy source that can be used today for transportation,energy storage and power generation simultaneously.The direct and rapid production of"green hydrogen"by water electrolysis has become a hot topic of research in hydrogen production.It consists of two electrochemical reactions,the hydrogen evolution reaction（HER）at the cathode and the oxygen evolution reaction（OER）at the anode.Driving OER requires a higher overpotential than HER,which stems from its slow four-electron-proton transfer process.Therefore,enhancing the electrocatalytic activity of OER to reduce the voltage required for electrocatalytic oxygen production reactions is a current endeavour of many researchers.In addition,the best options for OER catalysts are still precious metals Iridium and Ruthenium oxide,but their scarce reserves,high price and poor stability greatly increase the cost of water electrolysis technology for hydrogen production,limiting its commercial application.Therefore,the development of cheap,efficient and stable OER catalyst has important scientific value and research significance for the construction of hydrogen society in China.Transition metal oxides,especially cobalt-based materials,have good OER activity in alkaline environments.However,OER catalysts represented by cobalt tetraoxide have poor electrical conductivity,which increases the electrical energy to a certain extent;moreover,they are structurally unstable in the electrolyte and deactivate rapidly.On the other hand,it has been found in recent years that the lattice oxygen precipitation mechanism（LOM）has an intrinsically lower reaction potential than the conventional adsorbate conversion mechanism（AEM）,which is of more practical application and research significance.However,the potential barrier for lattice oxygen coupling precipitation in LOM cycles is often lower than the potential barrier for their recharge from the electrolyte,so the process of lattice oxygen removal/acquisition is often out of balance,resulting in an abrupt reduction in cycle life,which is a considerable bottleneck for its application.In this paper,Co₃O₄ is used as a research target,and strategies such as creating oxygen vacancies,doping with heterogeneous elements,encapsulating functional substrates and micro-controlling the crystal structure are adopted to improve the catalyst conductivity and increase the number of active sites.At the same time,this thesis focuses on the effect of OH-supply capacity on OER activity and stability under different mechanisms（AEM and LOM）.The main components include the following two parts:（1）Based on the AEM mechanism,the focus is on the effect of hydrophilic BO_xmotifs on the electrochemical OER parameters of Co₃O₄nanowire arrays.The researchers used Co₃O₄ to catalyse the hydrolysis reaction of NaBH₄ at room temperature while creating oxygen vacancies and mosaicking hydrophilic BO_x motifs（Co₃O₄/BO_x）inside and outside of it,respectively.By studying the changes in the electrochemical influences on Co₃O₄/BO_x during OER,it was found that the enhanced electrical conductivity and OH-supply capacity were the main reasons for the OER activity and stability of Co₃O₄/BO_x.It requires only 328 m V overpotential to drive a baseline current density of 10 m A cm^-2,which is significantly lower than that of pure Co₃O₄;and the activity does not decay significantly after 24 hours of continuous water electrolysis.This part of the study provides novel and straightforward strategies to regulate the catalyst conductivity synergistically,OH^-filling capacity,and the number of active centres,and is an essential guide to promote the practical application of the catalyst.（2）The paper further took a simple bimetallic spinel-type(Co_1-xNi_x)₃O₄,which can subtly drive the LOM mechanism by heat treatment and tuning the Ni stoichiometry ratio.At the same time,the NaBH₄ room temperature hydrolysis reaction creates oxygen vacancies and hydrophilic mosaic motifs inside and outside it.It was found that the crystal structure of NaBH₄ hydrolysis and heat-treated bimetallic spinel-type(Co_1-xNi_x)₃O₄ can be transformed to a rhombic structure,and this phase transition process transforms the OER mechanism of the catalyst from conventional AEM to a more advanced LOM,greatly enhancing the OER activity,which requires an overpotential of only 310 m V to drive a more significant current density of 20 m A cm^-2,achieving stability over 24 hours.The research results provide a novel strategy for guiding the mechanism of driving LOM in spinel-based materials and successfully overcoming the bottleneck of LOM-type catalysts’generally short cycle life by using the mosaic of hydrophilic substrates,which is of great practical and academic value.